This disclosure relates to an apparatus and method to avoid multiplexing of control and data for mobile users in a Long Term Evolution (LTE) reverse link. More particularly, this disclosure relates to a method and apparatus for coordinating the uplink and downlink scheduling assignments for power-limited users in the LTE reverse link.
While the disclosure is particularly directed to a particular telecommunications scheduling method to avoid multiplexing for power limited users, and thus will be described with specific reference thereto, it will be appreciated that this disclosure may have usefulness in other fields and applications. For example, this disclosure may be used in a variety of telecommunication networks where uplink and downlink scheduling assignments may be coordinated.
By way of background, 3rd Generation Partnership Project (3GPP) LTE has chosen Single Carrier Frequency Domain Multiple Access (SC-FDMA) for the reverse link. As a consequence, when control signaling is transmitted from the mobile terminal to the base station at the same time that there is data transmitted, then the control signaling must be multiplexed together with the data through appropriate rate matching of the data information. The rate matching results in puncturing of the coded data symbols in order to make space for the control channel signaling 17, 19, as shown in FIG. 1 and FIG. 2.
Examples of control information which needs to be sent in the reverse link in LTE include ACK/NACK information to support Hybrid Automatic Repeat reQuest (HARQ) in the forward link and Channel Quality Indication (CQI) which provides information to the base station 13 on the quality of the channel in the forward link. These two main types of control signaling are sent in the uplink in order to support the downlink.
ACK/NACK for HARQ—For every packet sent to the mobile in the downlink that the mobile detects, the mobile will generate a positive acknowledgement (ACK) if the packet was decoded successfully. The mobile will generate a negative acknowledgement (NACK) if the packet could not be decoded successfully. The mobile will transmit the ACK or NACK at a fixed time after the base station 13 transmitted the packet to the mobile. As shown in FIG. 1, if the downlink packet 15 was transmitted at subframe 0, the mobile 11 sends the ACK or NACK back to the base station 13, on the uplink, three subframes later at subframe 3 (note that the subframe duration in LTE is 1 millisecond). Note that this is but one example of the prior art, and the subframe duration, as well as the number of subframes between the downlink packet transmission and the mobile transmission of the ACK or NACK, may vary according to different embodiments.
CQI—CQI is a measurement of the downlink channel quality as measured by the mobile 11. The mobile 11 makes such a measurement and transmits it in the uplink back to the base station 13. The transmission of CQI information 19 is controlled by the base station 13 through higher layer of signaling whereby the base station sets up a starting time and a periodic reporting cycle. As shown in FIG. 2, CQI 19 is transmitted by the mobile 11 every 10 milliseconds, starting in subframe 2. In this embodiment, the CQI 19 is again reported in subframe 12 and so forth. This also is but one embodiment in the prior art, and this periodic reporting cycle may vary.
Puncturing the data symbols with control information increases the code rate on the data channel, which will reduce the Quality of Service (QoS) on the data channel. This, in turn, increases the error rate if no action is taken in order to compensate for the puncturing 21.
As shown in FIG. 3, it is useful to maintain the QoS on the data channel in the presence of control channel multiplexing. One popular approach is for the base station 13 to signal an additional power offset 31 to the mobile 11 to compensate for the puncturing 21 introduced from this control channel signaling 17, 19. As illustrated in FIG. 3, the signal travels through the Discrete Fourier Transform (DFT) 23, through subcarrier mapping 25, Inverse Fast Fourier Transform (IFFT) 27 and through cyclic prefix (CP) insertion 29, at a power of P+Δ control. P is the nominal transmit power setting the mobile 11 uses when there is no control channel multiplexing. P plus A control is the adjusted power setting when a specific type of control signaling (e.g. ACK/NACK 17, CQI 19, etc.) is multiplexed with the data 15. FIG. 3 illustrates the concept of applying additional power offset Δ control to the nominal power level P to compensate for the puncturing 21 of the data 15 with the control information 17, 19.
There are commonly two types of scheduling supported in the 3GPP LTE. These types of scheduling grants include dynamic scheduling and persistent scheduling. In dynamic scheduling, every packet transmission from the base station 13 to the mobile 11 (downlink) and the mobile 11 to the base station 13 (uplink) are explicitly scheduled by the base station 13 through the use of a scheduling grant. There is a separate scheduling grant for downlink transmissions and uplink transmissions. The scheduling grant is issued through the base station scheduler 35. This scheduling grant may be sent on the Physical Downlink Control CHannel (PDCCH). Persistent scheduling is generally used in order to alleviate control channel bottlenecks in the LTE. With persistent scheduling, a higher layer message (layer 2 or layer 3) informs the mobile 11 that it is scheduled at predetermined time instances and at predetermined locations in frequency. Through persistent scheduling, a specific packet size and modulation scheme may be used.
There is a separate persistent scheduling message for the uplink and the downlink. For example, a mobile 11 may be assigned a persistent allocation which allows it to transmit in the uplink or receive in the downlink every 5 milliseconds using 360 kilohertz of bandwidth starting at a frequency location with a modulation scheme of QPSK and a packet size of 320 bits. This type of scheduling is especially useful for supporting a large number of users which have a traffic source with a predictable arrival rate. Voice over Internet Protocol (VoIP) is one prominent example that often uses persistent scheduling.
One common problem with multiplexing in the prior art is that a mobile may already be transmitting at a maximum power in order to maintain the QoS on the uplink data channel in the absence of uplink control information. Stated another way, when the control information is multiplexed with the uplink data information, the solution in the current art of increasing the mobile transmit power is not applicable. For example, adding delta control to an already maxed out power is not feasible because the mobile is already operating at maximum power. In instances such as these, the QoS suffers due to this multiplexing solution.
There is a need in the industry for a method that permits data and control to be scheduled in a way that multiplexing is not necessary. There is also a need in the industry for a method and a system that provides the solution where uplink and downlink scheduling assignments are coordinated in such a way to avoid multiplexing specifically for power limited mobile users. It would also be useful for this solution to be equally feasible regardless or uplink and downlink scheduling grants.
The present disclosure contemplates a new and improved method that resolves the above-referenced difficulties and others.